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Proc. of 1st US-Italy Seismic Bridge Workshop, EUCENTRE, Pavia, Italy 18 - 20 April 2007. SELECTION OF TIME-HISTORIES FOR BRIDGE DESIGN IN EUROCODE 8 IUNIO IERVOLINO, GIUSEPPE MADDALONI, EDOARDO COSENZA AND GAETANO MANFREDI Dipartimento di Ingegneria Strutturale, Università degli Studi di Napoli Federico II Abstract: Eurocode 8 (EC8) allows the use of real earthquake records as an input for time-history analysis of structures. In its Part 2, the code discusses the preparation of seismic input for bridge structures; although referring to the same design spectral shapes of Part 1, the prescriptions for bridges are somewhat differently specified in respect to those for buildings. The main requirement the chosen record set should satisfy, is the compatibility of the horizontal average SRSS spectrum with the design spectrum in a broad range of periods. The set has to be made of at least three recordings, but seven is the minimum set size to consider the mean structural response as the design value. The code seems to indicate real records as the principal source of ground motions the practitioner should rely on, however the selection of real records is not straightforward and requires searching of large databases to find sets compliant with the code spectrum. In another study, the authors discussed the record selection prescriptions in EC8 Part 1, herein these issues are reviewed in respect to the specific requirements given in EC8 Part 2, and the European Strong-motion Database is searched to identify real record sets matching the design spectral shapes for several site conditions. Key words: Time-history, Eurocode 8, Records, Bridges. 1. INTRODUCTION The availability of on-line, user-friendly, databases of strong-motion recordings and the rapid development of digital seismic networks worldwide have increased the accessibility to natural accelerograms. This is the main reason why they currently seem to be the most attractive option to define the seismic input in order to assess seismic structural 2 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi performance. Therefore, earthquake engineering research has focused lately on the selection of real ground motions for non-linear structural analysis, and relatively simple procedures have been developed to link records to the hazard at the site [Cornell, 2004]. In spite this effort, in many seismic codes the guidelines about preparation of ground motion input for dynamic analysis are poor, especially in those cases when bi-directional loading is required [Beyer and Bommer, 2007]. This is partially because research on the topic is developing fast and at least a few years are required by regulations to take it in. Norms are similar worldwide, the minimum set size is typically 3 to 7 records, and the main prescription is the compatibility with the design spectrum in a specified range of periods. Eurocode 8 (EC8) Part 1 [CEN, 2003] asks that the average spectral ordinates of the chosen set should match the design spectrum with some tolerance. Little, if any, prescriptions are given about other features of the signal. EC8 Part 2 [CEN, 2005], referring to bridges, has similar prescriptions but more detailed; it refers to the same spectral shapes and site conditions given in Part 1 and still requires three-dimensional seismic input, but it specifies that the matching with the design spectrum should be carried out considering the square root summation of squares (SRSS) of the two horizontal spectra independently of the vertical component. Other particular conditions about near source and spatial variability of seismic motion are also specifically considered in Part 2. EC8, in preparation of seismic input, requires consistency of the chosen record set with the specific features of the seismic sources the site of interest is subjected to, however the spectral shapes given are related to the hazard only via the anchoring value (ag). EC8 does not provide anchor values for its non-dimensional spectral shapes. The ag values considered herein correspond to the Italian case, where the seismic territory is divided into four zones representing different hazard levels, where seismic resistant design is mandatory only in the upper three zones1. In another study [Iervolino et al., 2007] the authors discuss the EC8 Part 1 requirements for real accelerograms in respect to the best current practice in record selection. In the same study, the European Strong-motion Database (ESD) is investigated to find real record sets complying as much as possible with EC8 spectra. Herein the EC8 Part 2 prescriptions are considered. The ESD dataset has been investigated in order to find un- scaled (original) record sets respecting as much as possible the spectral matching requirements. Moreover, sets of scaled code-compatible accelerograms were also considered in order to reduce the record-to-record variability of the spectra, and to obtain sets which are independent of the anchoring value of the code spectrum. Finally, sets are found and some examples are shown herein. 1 The ag values for the Zone 1, 2 and 3 are 0.15g, 0.25g and 0.35g respectively. In fact, the if the peak ground acceleration (on rock) with a 10% exceeding probability in 50 years falls in one of the intervals ]0.25g, 0.35g], ]0.15g, 0.25g], or ]0.05g, 0.15g], then the site is classified as Zone 1, 2 or 3 respectively [OPCM 3519, 2006]. 1st US-Italy Seismic Bridge Workshop 3 2. SEISMIC INPUT FOR NON-LINEAR DYNAMIC ANALYSIS IN EUROCODE 8 PART 2 In EC8 the seismic input for time-history analysis is defined after the elastic response spectrum. In Part 1 the spectral shapes are given for both horizontal and vertical components of motion. Section 3.2.22 gives two spectral shapes defined as type 1 and type 2. The latter applies if the earthquake contributing most to the seismic hazard has surface waves magnitude not grater that 5.5, otherwise the former should be used. The design spectral shape for the vertical component has the same distinction. In Figure 1 the 5% damped elastic spectra for the five main site classes are given as normalized in respect to ag. The anchoring value for the vertical elastic spectrum (avg) is defined by the suggested ratio avg/ag=0.9. EC8 Part 2 refers to the same spectral shapes in order to define the seismic input for time-history analysis of bridges. Figure 1. EC8 horizontal and vertical type 1 spectral shapes 2.1 General requirements and spectral matching The requirements for the seismic input for dynamic analysis are given in section 3.2.3: when a non-linear time-history analysis is carried-out, at least three pairs of horizontal ground motion time- history components shall be used. The pairs should be selected from recorded events with magnitudes, source distances, and mechanisms consistent with those that define the design seismic action. When the required number of pairs of appropriate recorded ground motions is not available, appropriate modified recordings or simulated accelerograms may replace the missing recorded motions. The chosen set of accelerograms, should match the following criteria: 2 In the rest of the paper all calls and verbatim citations of Eurocode 8 will be simply indicated in italic. 4 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi a) for each earthquake consisting of a pair of horizontal motions, the SRSS spectrum shall be established by taking the square root of the sum of squares of the 5%-damped spectra of each component; b) the spectrum of the ensemble of earthquakes shall be formed by taking the average value of the SRSS spectra of the individual earthquakes of the previous step. c) The ensemble spectrum shall be scaled so that it is not lower than 1,3 times the 5% damped elastic response spectrum of the design seismic action3, in the period range between 0,2T1 and 1,5T1, where T1 is the natural period of the fundamental mode of the structure in the case of a ductile bridge, or the effective period (Teff) of the isolation system in the case of a bridge with seismic isolation. d) When the SRSS spectrum of the components of a recorded accelerogram gives accelerations the ratio of which to the corresponding values of the elastic response spectrum of the design seismic action shows large variation in the period range in (c), modification of the recorded accelerogram may be carried out, so that the SRSS spectrum of the modified components is in closer agreement with the elastic response spectrum of the design seismic action. e) When three component ground motion time-history recordings are used for nonlinear time- history analysis, scaling of the horizontal pairs of components may be carried out in accordance with previous step, independently from the scaling of the vertical components. The latter shall be effected so that the average of the relevant spectra of the ensemble is not lower by more than 10% of the 5% damped elastic response spectrum of the vertical design seismic action in the period range between 0,2Tv and 1,5Tv, where Tv is the period of the lowest mode where the response to the vertical component prevails over the response to the horizontal components (e.g. in terms of participating mass). f) The use of pairs of horizontal ground motion recordings in combination with vertical recordings of different seismic motions, is also allowed. The independent scaling of the pairs of horizontal recordings and of the vertical recordings shall be carried out. In section 4.2.4.3, the code allows the consideration of the mean effects on the structure, rather then the maximum, when non-linear dynamic analysis is performed for at least seven independent ground motions. 3 EC8 Part 1 specifies that the chosen set should not underestimate the code spectrum more than 10%. The prescription of EC8 Part 2 seems to be equivalent because the SRSS, for equal spectra in the two directions, is exactely one of the two spectra times 1.4. 1st US-Italy Seismic Bridge Workshop 5 2.2 Vertical component of seismic action and other issues in record selection As specified in section 4.1.7, the effects of the vertical seismic component on the piers may be omitted in cases of low and moderate seismicity, while in zones of high seismicity these effects need only be taken into account in the following conditions: (a) if the piers are subjected to high bending stresses due to vertical permanent actions of the deck; (b) when the bridge is located within 5 km of an active seismotectonic fault; (c) for a prestressed concrete deck; (d) the effects of the vertical seismic component on bearings and links shall always be taken into account. EC8 Part 2 has specific prescriptions for special cases as near-source conditions. In particular, the code prescribes that site-specific spectra considering near source effects shall be used, when the site is located horizontally within 10 km of a known active seismotectonic fault that may produce an event of Moment Magnitude higher than 6.5. The code also prescribes when the spatial variability of ground motion has to be considered (section 3.3): when the soil properties along the bridge vary to the extent that more than one ground types correspond to the supports of the bridge deck; or when soil properties along the bridge are approximately uniform, but the length of the continuous deck exceeds the distance beyond which the ground motions may be considered uncorrelated. These special cases were not considered in the present study. 3. SELECTION CRITERIA CONSIDERED Herein it was not possible to select natural records from events having source features consistent with those that define the design seismic action as prescribed in section 2, because this study does not refer to any site-specific case. However, it has to be noted that the design spectrum is related to the seismic hazard at the site only through the ag value, which is often provided by the national authorities and it is not common for engineers involved in design to have source parameters to match. Other spectral matching requirements given in section 2 were considered along with additional criteria, which allowed to rank the record sets found. The additional criteria considered are: a) the deviation of the average spectrum in respect to the code spectrum (δ), which gives a measure of how much the mean spectrum of a records’ combination deviates from the spectrum of the code. Definition of δ is given in Eq. (1), where: Sao,med (Ti ) is the ordinate of the average spectrum corresponding to the period Ti; Sas (Ti ) is the value of 1.3 [1.0] times the ordinate of the code spectrum at the same period horizontal [vertical] spectrum; and N is the number of values within the considered range of periods. Selecting a record set with a low δ may allow to obtain an average spectrum which is well approximating the code. The deviation δ was computed separately for the horizontal SRSS and the vertical components of motion, and indicated as δH and δV respectively. 6 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi ( T ) − Sa ( T ) ⎞ 2 1 N ⎛ Sa δ= N ∑ ⎜ o ,med Sai (T ) s i ⎟ ⎜ i =1 ⎝ ⎟ (1) s i ⎠ b) The maximum deviation of a single spectrum within a set in respect to the code spectrum (δmax), which is defined as in Eq. (1) replacing Sao,med (Ti ) with the ordinates of a single spectrum of the combination. Controlling this parameter may allow choosing combinations characterized by records having the individual spectra relatively close to the reference spectrum, and therefore being narrowly distributed around it. c) The number of different events within a set. This criterion corresponds to the identification of combinations of records featuring the largest number of different events possible. 4. WAVEFORM DATABASE AND ANALYSES The investigated database is the European Strong-motion Database, whose URL is http://www.isesd.cv.ic.ac.uk; see Ambraseys et al. [2000, 2004] for further information. Selecting the recording stations by the site class (rock, stiff soil, soft soil, very soft soil, and alluvium) allows to download all the spectra of accelerograms recorded on each ground category. The website was accessed in April 2005 (April 2007 for A site class). Stations without all three translational components were discarded. Moreover, only events characterized by a Moment Magnitude equal or larger than 5.8 (6.0 for site class A) have been retained. For D site class, no reduction to the initial record list was made because of the shortage of stations for that geological condition. The resulting numbers of horizontal records are given in Table 1, where the two horizontal and vertical components are conventionally indicated as X, Y and Z respectively. Table 1. Input dataset considered in the study Local Geology X Y Z A (rock) 134 134 134 B (stiff soil) 134 134 134 C (soft soil) 122 122 122 D (very soft soil) 28 28 28 E (alluvium) 29 29 29 The identified database of records was investigated, via a specifically developed computer code, in order to find sets that consist of 7 earthquakes (sets made of 7 pairs considering the SRSS of the 5% damped spectra of both horizontal components, and the 7 vertical components), for all five site classes. The code analyzed all possible combinations of 1st US-Italy Seismic Bridge Workshop 7 spectra of the input list checking the match with the code shapes. The compatibility interval was chosen to be 0.139 sec - 4 sec for horizontal and 0.04 sec – 2 sec for vertical components. These intervals, according to condition (c) and (e) of section 2, render the record sets found suitable for structures with horizontal fundamental period T1 in the range 0.7 sec – 2.7 sec and for structures with vertical fundamental period Tv in the range 0.2 sec – 1.3 sec. For each combination, the software also computes the deviation of each single spectrum within the set, and also the deviation of the average spectrum from the code spectrum. Results of this search were manually 4 ranked with respect to the additional criteria given in the previous section. In the following the total number of sets compatible with EC8 spectra is presented and selected results, referring to both un-scaled (original) and normalized (non-dimensional) records are displayed. It has to be anticipated that for D and E soil types, no results were found. This is primarily due to the shortage of earthquake recordings on these soils in the database. 4.1 Sets of un-scaled (original) spectra The chosen dataset has been investigated for sets made of 7 un-scaled SRSS horizontal spectra. The average has been calculated on seven spectra, obtained from 14 original records, by taking the square root of the sum of squares of both X and Y components of seven recording stations. A summary of results is given in Table 2, which shows, for any site class and seismic zone for which combinations exist, the number of spectrum- compatible sets found and the corresponding tolerance in matching the average spectrum imposed in the analyses (it is to recall here that the compatibility lower bound assigned by EC8 is 30% above the code spectrum). The compatibility upper bound was adjusted adaptively find more Eurocode spectra compatible sets. As it is expected, the larger number of results corresponds to the lower hazard levels. It should also be noted that for the higher hazard levels of soil C, it was not possible to find results satisfying the EC8 prescriptions5. In fact, the lower bound had to be relaxed; sets found in this case may be linearly scaled to comply with the code spectrum. For those combinations found and listed in Table 2, it has been verified whether the sets made of vertical components match the code spectrum, without scaling. Results of this analysis are given in Table 3. For A and C ground types, it was not possible to find results satisfying the spectral matching requirements. For this reason, the lower bound was relaxed; again, sets found may be linearly scaled to comply with the code spectrum. 4Another more refined option to carry out this job via genetic algorithms is that proposed by Naeim et al. [2004]. 5 The compatibility lower bounds for Zone 1 of C site class are given in Table 2; 0% above the code spectrum is the minimum tolerance to find results. These levels have been obtained iteratively reducing (with a 10% step) the lower bound in the analyses. In other words it was not possible to find suitable results using 30%, 20% and 10% respectively. The upper bounds have been chosen arbitrarily to obtain a significant number of resulting record sets. 8 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi Table 2. Compliant horizontal SRSS sets found Ground Zone Compatibility Lower Bound6 Compatibility Upper Bound6 Sets Found A 1 30% 1000% 4 2 30% 100% 127177 3 30% 70% 380385 B 1 30% 1000% 53 2 30% 80% 4179 3 30% 80% 838263 C 1 0% 1000% 10 2 30% 100% 167 3 30% 80% 115307 Table 3. Vertical components’ sets found Ground Zone Compatibility Lower Bound7 Compatibility Upper Bound6 Sets Found A 1 25% 1000% 3 2 30% 1000% 18 3 40% 1000% 226 B 1 10% 1000% 7 2 10% 1000% 41 3 10% 1000% 8207 C 1 55% 1000% 3 2 30% 1000% 1 3 30% 1000% 99 As an example, selected results are given from Figure 2 to Figure 10. They correspond to all three hazard levels of all site classes. In the figures the rough thick curve represents the average spectrum and the thick smooth curve represents the code spectrum. The dashed line is the compatibility limit prescribed by the code; thin lines are the individual spectra within a set. In the legend of any figure the seven digits station code, as well as the earthquake code (EQ) from the ESD database, is given. The legend, when needed, also reports the value of scale factors to comply with EC8 spectra. For details about the records displayed see the Appendix. 6 Expressed in terms of overestimation of the code spectrum. 7 Expressed in terms of underestimation of the code spectrum. 1st US-Italy Seismic Bridge Workshop 9 Horizontal SRSS 000055 EQ:34 1.5 000182 EQ:87 000198 EQ:93 000290 EQ:146 006332 EQ:2142 Sa [g] 1 006349 EQ:2142 007142 EQ:2309 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000055 EQ:34 SF:1.65 1.5 000182 EQ:87 SF:1.65 000198 EQ:93 SF:1.65 000290 EQ:146 SF:1.65 006332 EQ:2142 SF:1.65 Sa [g] 1 006349 EQ:2142 SF:1.65 007142 EQ:2309 SF:1.65 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 2. Site class A – Zone 1. Set with minimum average deviation from the horizontal target spectrum (δH = 0.235). SF is the individual scale factor. Horizontal SRSS 000055 EQ:34 1.5 000182 EQ:87 000198 EQ:93 000200 EQ:93 001228 EQ:472 Sa [g] 1 004674 EQ:1635 007142 EQ:2309 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000055 EQ:34 SF:1.35 1.5 000182 EQ:87 SF:1.35 000198 EQ:93 SF:1.35 000200 EQ:93 SF:1.35 001228 EQ:472 SF:1.35 Sa [g] 1 004674 EQ:1635 SF:1.35 007142 EQ:2309 SF:1.35 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 3. Site class A – Zone 2. Set with minimum single-record deviation from the horizontal and vertical target spectrum (δHmax = 0.808; δVmax = 0.513) and minimum average deviation from the vertical target spectrum (δV = 0.231). SF is the individual scale factor. 10 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi Horizontal SRSS 000052 EQ:34 1.5 000182 EQ:87 000198 EQ:93 000200 EQ:93 001255 EQ:472 Sa [g] 1 004679 EQ:1635 006335 EQ:2142 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000052 EQ:34 SF:1.5 1.5 000182 EQ:87 SF:1.5 000198 EQ:93 SF:1.5 000200 EQ:93 SF:1.5 001255 EQ:472 SF:1.5 Sa [g] 1 004679 EQ:1635 SF:1.5 006335 EQ:2142 SF:1.5 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 4. Site class A – Zone 3. Set with minimum single-record deviation from the horizontal and vertical target (δHmax = 1.336; δVmax = 0.799). SF is the individual scale factor. Horizontal SRSS 000187 EQ:87 1.5 000196 EQ:93 000197 EQ:93 000199 EQ:93 000228 EQ:108 Sa [g] 1 006263 EQ:1635 006334 EQ:2142 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000187 EQ:87 1.5 000196 EQ:93 000197 EQ:93 000199 EQ:93 000228 EQ:108 Sa [g] 1 006263 EQ:1635 006334 EQ:2142 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 5. Site class B – Zone 1. Set with minimum average deviation from the horizontal and vertical target spectrum (δH = 0.221; δV = 0.171). 1st US-Italy Seismic Bridge Workshop 11 Horizontal SRSS 000197 EQ:93 1.5 000199 EQ:93 000228 EQ:108 000231 EQ:108 004673 EQ:1635 Sa [g] 1 006263 EQ:1635 006334 EQ:2142 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000197 EQ:93 1.5 000199 EQ:93 000228 EQ:108 000231 EQ:108 004673 EQ:1635 Sa [g] 1 006263 EQ:1635 006334 EQ:2142 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 6. Site class B – Zone 2. Set with minimum average deviation from vertical target spectrum (δV = 0.175). Horizontal SRSS 000181 EQ:87 1.5 000197 EQ:93 000230 EQ:108 000536 EQ:250 002015 EQ:169 Sa [g] 1 004673 EQ:1635 006328 EQ:2142 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000181 EQ:87 1.5 000197 EQ:93 000230 EQ:108 000536 EQ:250 002015 EQ:169 Sa [g] 1 004673 EQ:1635 006328 EQ:2142 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 7. Site class B – Zone 3. Set with minimum average deviation from the vertical target spectrum (δV = 0.338) with records belonging from seven different events. 12 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi Horizontal SRSS 000042 EQ:30 SF:1.8 1.5 000879 EQ:349 SF:1 007329 EQ:2343 SF:1.4 001226 EQ:472 SF:1.5 001257 EQ:472 SF:1.3 Sa [g] 1 001560 EQ:497 SF:1 001703 EQ:497 SF:1 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000042 EQ:30 SF:1.85 1.5 000879 EQ:349 SF:1.85 007329 EQ:2343 SF:1.85 001226 EQ:472 SF:1.85 001257 EQ:472 SF:1.85 Sa [g] 1 001560 EQ:497 SF:1.85 001703 EQ:497 SF:1.85 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 8. Site class C – Zone 1. Set with minimum single-record deviation from the horizontal target spectrum (δHmax = 0.525) and minimum average deviation from the vertical target spectrum (δV = 0.125). SF is the individual scale factor. Horizontal SRSS 000879 EQ:349 1.5 007329 EQ:2343 001226 EQ:472 001257 EQ:472 006959 EQ:473 Sa [g] 1 001560 EQ:497 001703 EQ:497 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000879 EQ:349 SF:1.75 1.5 007329 EQ:2343 SF:1 001226 EQ:472 SF:1 001257 EQ:472 SF:1 006959 EQ:473 SF:1.7 Sa [g] 1 001560 EQ:497 SF:1.8 001703 EQ:497 SF:1 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 9. Site class C – Zone 2. Unique un-scaled set found. SF is the individual scale factor. 1st US-Italy Seismic Bridge Workshop 13 Horizontal SRSS 000151 EQ:65 1.5 000479 EQ:230 001726 EQ:561 007329 EQ:2343 001226 EQ:472 Sa [g] 1 001230 EQ:472 006959 EQ:473 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Vertical 000151 EQ:65 SF:1.75 1.5 000479 EQ:230 SF:2 001726 EQ:561 SF:1.3 007329 EQ:2343 SF:1 001226 EQ:472 SF:1 Sa [g] 1 001230 EQ:472 SF:1.8 006959 EQ:473 SF:1.45 0.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period [sec] Figure 10. Site class C – Zone 3. Set with minimum single-record deviation from the horizontal and vertical target spectrum (δHmax = 1.274; δVmax = 1.163) and minimum average deviation from the horizontal target spectrum (δH = 0.147). SF is the individual scale factor. 4.2 Normalized (non-dimensional) sets The input database has also been examined for records normalized, which should allow to select records having a spectral shape similar to that of the code. However, this entails scaling the records, which was avoided, if possible, in the analyses presented in the previous sections. Here, the records have been rendered non-dimensional by dividing the spectral ordinates by their spectral acceleration at T = 0.04 sec (assumed to be an approximation of the peak ground acceleration). After normalizing the spectra, the same analyses discussed above have been repeated. In fact, combinations of these spectra have been compared to the non-dimensional code spectra, and then the results (summarized in Table 4) may be considered country-independent because they’re independent of the ag values. From Figure 11 to Figure 13 some results for the horizontal SRSS components are given for A, B and C site classes. As expected normalization of spectra reduces the spectral variability within a set while keeping a good average matching with the code if large scale factors (given in Table 5) are allowed. Table 4. Number of normalized (non-dimensional) compliant sets found Ground Compatibility Lower Bound7 Compatibility Upper Bound6 Sets Found A 10% 100% 9053 B 10% 100% 53465 C 10% 100% 8139 14 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi Horizontal SRSS;Non−dimensional 4 000172 EQ:81 000286 EQ:146 001255 EQ:472 3 002553 EQ:239 Sa [adim] 004674 EQ:1635 005821 EQ:1888 2 007115 EQ:2302 1 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Figure 11 – Site class A. Set with minimum average deviation from the horizontal target spectrum (δH = 0.092) with records belonging from seven different events. Horizontal SRSS;Non−dimensional 4 000232 EQ:108 000500 EQ:239 002019 EQ:709 3 002030 EQ:169 Sa [adim] 005809 EQ:1887 006142 EQ:559 2 007143 EQ:2309 1 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Figure 12 – Site class B. Set with minimum average deviation from the horizontal target spectrum (δH = 0.066) with records belonging from seven different events. Horizontal SRSS;Non−dimensional 005673 EQ:83 4 000175 EQ:83 000555 EQ:264 3 001251 EQ:472 Sa [adim] 006961 EQ:473 006963 EQ:473 2 006967 EQ:473 1 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period [sec] Figure 13 – Site class C. Set with minimum average deviation from the horizontal target spectrum (δH = 0.079). Table 5. Average scaling factor to match the code spectra of non-dimensional records Site/Zone ag SF A-1 0.35 11.6 A-2 0.25 8.3 A-3 0.15 5.0 B-1 0.35 24.2 B-2 0.25 17.3 B-3 0.15 10.4 1st US-Italy Seismic Bridge Workshop 15 C-1 0.35 10.8 C-2 0.25 7.7 C-3 0.15 4.6 5. CONCLUSIONS In the presented study the EC8 Part 2 prescriptions for selection of the seismic input for non-linear time-history analysis of bridges have been reviewed. The code requires the use of real records consistent with magnitude, distance and faulting mechanism driving the seismic hazard. The chosen record set should have the average of the SRSS larger than 1.3 times the code spectrum, while the average of the vertical components (to be considered in special cases only) should be not lower than 0.9 times the vertical code spectrum in a certain range of periods. The European Strong-motion database was investigated to search for record sets matching the EC8 Part 2 spectral requirements. The analyses performed did not try to match any specific magnitude, distance and mechanism because this study do not refer to any site-specific application, however it has to be noted that the code spectrum is only indirectly related to seismic hazard at the site, and hazard disaggregation parameters are not always available to engineers. The exercise performed allowed to find results, some of which are presented herein. The sets are of two types: (1) combinations made of records which do not require scaling to match the code spectrum compatibility requirement; (2) records to be scaled to comply with the code spectral prescription, which reduces the variability within a set at the price of eventually high scaling factors. ACKNOWLEDGEMENTS The study presented in this paper was developed within the activities of Rete dei Laboratori Universitari di Ingegneria Sismica – ReLUIS for the research program founded by Dipartimento di Protezione Civile. REFERENCES Ambraseys, N., Smit, P., Berardi, R., Rinaldis, D., Cotton, F. and Berge, C. [2000], Dissemination of European Strong-Motion Data (CD-ROM collection). European Commission, DGXII, Science, Research and Development, Bruxelles. Ambraseys, N.N., Douglas, J., Rinaldis, D., Berge-Thierry, C., Suhadolc, P., Costa, G., Sigbjornsson, R. and Smit, P. [2004], Dissemination of European strong-motion data, Vol. 2, CD- ROM Collection, Engineering and Physical Sciences Research Council, United Kingdom. 16 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi Beyer, K., Bommer, J.J. [2007], “Selection and scaling of real accelerograms for bi-directional loading: a review of current practice and code provisions,” Journal of Earthquake Engineering, 11(Supplement 1), 13-45. Cornell, C.A. [2004] “Hazard, Ground motions and Probabilistic assessment for PBSD,” In Performance based seismic design concepts and implementation. PEER Report 2004/05. Pacific Earthquake Engineering Research Center University of California Berkeley, CA, USA. CEN [2003] Eurocode 8: Design of structures for earthquake resistance. Part 1: General rules, seismic actions and rules for buildings, Final Draft prEN 1998, European Committee for Standardization, Brussels, Belgium. CEN [2005] Eurocode 8: Design of structures for earthquake resistance. Part 2: Bridges”, Final Draft prEN 1998-2, European Committee for Standardization, Brussels, Belgium. Iervolino, I., Maddaloni G., Cosenza E. [2007], “Eurocode 8 compliant real record sets for seismic analysis of structures,” Journal of Earthquake Engineering, in press. Naeim F., Alimoradi A., Pezeshk S. [2004] “Selection and scaling of ground motion time histories for structural design using genetic algorithms,” Earthquake Spectra, 20(2): 413–426. Ordinanza del Presidente del Consiglio dei Ministri (OPCM) n. 3519 [2006] “Criteri per l’individuazione delle zone sismiche e la formazione e l’aggiornamento degli elenchi delle medesime zone,” Gazzetta Ufficiale della Repubblica Italiana, 108. APPENDIX In this appendix the details about the records presented in the paper are given as they come from the European Strong-motion Database website. Table 6. A-type site class: record information for the un-scaled type sets (Figure 2 to Figure 4) Site/Zone Code Event Name Country Date Station Name A-1 000055 Friuli Italy 06/05/1976 Tolmezzo-Diga Ambiesta 000182 Tabas Iran 16/09/1978 Dayhook Serbia & 000198 Montenegro 15/04/1979 Ulcinj-Hotel Albatros Montenegro 000290 Campano Lucano Italy 23/11/1980 Sturno South Iceland 006332 Iceland 21/06/2000 Thjorsartun (aftershock) South Iceland 006349 Iceland 21/06/2000 Thjorsarbru (aftershock) 007142 Bingol Turkey 01/05/2003 Bingol-Bayindirlik Murlugu A-2 000055 Friuli Italy 06/05/1976 Tolmezzo-Diga Ambiesta 000182 Tabas Iran 16/09/1978 Dayhook 000198 Montenegro Serbia & 15/04/1979 Ulcinj-Hotel Albatros 1st US-Italy Seismic Bridge Workshop 17 Montenegro Serbia & Hercegnovi Novi-O.S.D. 000200 Montenegro 15/04/1979 Montenegro Pavicic School Gebze-Tubitak Marmara 001228 Izmit Turkey 17/08/1999 Arastirma Merkezi 004674 South Iceland Iceland 17/06/2000 Flagbjarnarholt 007142 Bingol Turkey 01/05/2003 Bingol-Bayindirlik Murlugu A-3 000052 Friuli Italy 06/05/1976 Feltre 000182 Tabas Iran 16/09/1978 Dayhook Serbia & 000198 Montenegro 15/04/1979 Ulcinj-Hotel Albatros Montenegro Serbia & Hercegnovi Novi-O.S.D. 000200 Montenegro 15/04/1979 Montenegro Pavicic School 001255 Izmit Turkey 17/08/1999 Heybeliada-Senatoryum 004679 South Iceland Iceland 17/06/2000 Hveragerdi-Retirement House South Iceland 006335 Iceland 21/06/2000 Selfoss-City Hall (aftershock) Table 7. B-type site class: record information for the un-scaled type sets (Figure 5 to Figure 7) Site/Zone Code Event Name Country Date Station Name B-1 000187 Tabas Iran 16/09/1978 Tabas 000196 Montenegro Yugoslavia 15/04/1979 Petrovac-Hotel Oliva 000197 Montenegro Yugoslavia 15/04/1979 Ulcinj-Hotel Olimpic 000199 Montenegro Yugoslavia 15/04/1979 Bar-Skupstina Opstine 000228 Montenegro (aftershock) Yugoslavia 24/05/1979 Bar-Skupstina Opstine 006263 South Iceland Iceland 17/06/2000 Kaldarholt 006334 South Iceland (aftershock) Iceland 21/06/2000 Solheimar B-2 000197 Montenegro Yugoslavia 15/04/1979 Ulcinj-Hotel Olimpic 000199 Montenegro Yugoslavia 15/04/1979 Bar-Skupstina Opstine 000228 Montenegro (aftershock) Yugoslavia 24/05/1979 Bar-Skupstina Opstine 000231 Montenegro (aftershock) Yugoslavia 24/05/1979 Tivat-Aerodrom 004673 South Iceland Iceland 17/06/2000 Hella 006263 South Iceland Iceland 17/06/2000 Kaldarholt 006334 South Iceland (aftershock) Iceland 21/06/2000 Solheimar B-3 000181 Tabas Iran 16/09/1978 Boshroyeh 000197 Montenegro Yugoslavia 15/04/1979 Ulcinj-Hotel Olimpic 18 Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi 000230 Montenegro (aftershock) Yugoslavia 24/05/1979 Budva-PTT Tercan-Meteoroji 000536 Erzincan Turkey 13/03/1992 Mudurlugu 002015 Kefallinia (aftershock) Greece 23/03/1983 Argostoli-OTE Building 004673 South Iceland Iceland 17/06/2000 Hella 006328 South Iceland (aftershock) Iceland 21/06/2000 Kaldarholt Table 8. C-type site class: record information for the un-scaled type sets (Figure 8 to Figure 10) Site/Zone Code Event Name Country Date Station Name C-1 000042 Ionian Greece 04/11/1973 Lefkada-OTE Building 000879 Dinar Turkey 01/10/1995 Dinar-Meteoroloji Mudurlugu 007329 Faial Portugal 09/07/1998 Horta 001226 Izmit Turkey 17/08/1999 Duzce-Meteoroloji Mudurlugu 001257 Izmit Turkey 17/08/1999 Yarimca-Petkim Bolu-Bayindirlik ve Iskan 001560 Duzce 1 Turkey 12/11/1999 Mudurlugu 001703 Duzce 1 Turkey 12/11/1999 Duzce-Meteoroloji Mudurlugu C-2 000879 Dinar Turkey 01/10/1995 Dinar-Meteoroloji Mudurlugu 007329 Faial Portugal 09/07/1998 Horta 001226 Izmit Turkey 17/08/1999 Duzce-Meteoroloji Mudurlugu 001257 Izmit Turkey 17/08/1999 Yarimca-Petkim 006959 Izmit (aftershock) Turkey 13/09/1999 Adapazari Bahtiyat Topcu Evi Bolu-Bayindirlik ve Iskan 001560 Duzce 1 Turkey 12/11/1999 Mudurlugu 001703 Duzce 1 Turkey 12/11/1999 Duzce-Meteoroloji Mudurlugu C-3 000151 Friuli (aftershock) Italy 15/09/1976 Buia 000479 Manjil Iran 20/06/1990 Rudsar 001726 Adana Turkey 27/06/1998 Ceyhan-Tarim Ilce Mudurlugu 007329 Faial Portugal 09/07/1998 Horta 001226 Izmit Turkey 17/08/1999 Duzce-Meteoroloji Mudurlugu 001230 Izmit Turkey 17/08/1999 Iznik-Karayollari Sefligi Muracaati 006959 Izmit (aftershock) Turkey 13/09/1999 Adapazari Bahtiyat Topcu Evi 1st US-Italy Seismic Bridge Workshop 19 Table 9. Record information for the non-dimensional type sets (Figure 11 to Figure 13) Site Code Event Name Country Date Station Name A 000172 Basso Tirreno Italy 15/04/1978 Messina 1 000286 Campano Lucano Italy 23/11/1980 Arienzo 001255 Izmit Turkey 17/08/1999 Heybeliada-Senatoryum 002553 Racha Georgia 29/04/1991 Toros 004674 South Iceland Iceland 17/06/2000 Flagbjarnarholt 005821 Strofades (aftershock) Greece 18/11/1997 Koroni-Town Hall (Library) 007115 Pulumur Turkey 27/01/2003 Bingol-Bayindirlik Murlugu B 000232 Montenegro (aftershock) Yugoslavia 24/05/1979 Kotor-Naselje Rakite 000500 Racha Georgia 29/04/1991 Bogdanovka Off coast of Magion Oros 002019 peninsula Greece 06/08/1983 Ierissos-Police Station 002030 Kefallinia (aftershock) Greece 23/03/1983 Zakynthos-OTE Building 005809 Strofades Greece 18/11/1997 Argostoli-OTE Building 006142 Aigion Greece 15/06/1995 Patra-San Dimitrios Church Elazig-Bayindirlik ve Iskan 007143 Bingol Turkey 01/05/2003 Mudurlugu C 005673 Volvi Greece 20/06/1978 Pehcevo-Duvanski Kombinat 000175 Volvi Greece 20/06/1978 Thessaloniki-City Hotel 000555 Kallithea Greece 18/03/1993 Patra-OTE Building 001251 Izmit Turkey 17/08/1999 Bursa-Tofa Fabrikasi 006961 Izmit (aftershock) Turkey 13/09/1999 Adapazari A.Babalioglu Evi 006963 Izmit (aftershock) Turkey 13/09/1999 Akyazi Gebes Koyu Imam Lojmani 006967 Izmit (aftershock) Turkey 13/09/1999 Ambarli-Termik Santrali